1. Field of the Invention
This invention relates generally to the detection of buried metallic objects, and more specifically to a novel apparatus to be used for the more accurate determination of the depth, size, and shape of buried metallic objects indicative of unexploded ordnance.
In land areas where ordnance has been used, such as artillery ranges, abandoned battle fields and the like, the reclamation of such lands for public or private use is problematic given the oftentimes presence of buried, unexploded ordnance, which ordnance can be triggered in the course of land grading, excavation and the like to thus present unacceptable explosion hazards to those in the field. Presently, metal detectors alone provide less than an ideal means for detection of such ordnance as with the current state of the art detectors have difficulty distinguishing between exploded ordnance which presents itself as scraps or fragments of metal, and unexploded ordnance which presents itself as a projectile in its original form such as of an ellipsoid shape.
2. Description of the Related Art
In our earlier filed provisional application Ser. No. 60/975,911 and our earlier provisional application Ser. No. 60/740,576 filed Nov. 28, 2005, we described apparatus for the detection of UXOs (unexploded ordnance) combining several orthogonally arranged transmission coils with a plurality of receiving coils disposed symmetrically within the coil space, as illustrated in the figures of those applications, and a related article entitled Multi-transmitter Multi-receiver Null Coupled Systems for Inductive Detection and Characterization of Metallic Objects, Smith et. al., Journaled of Applied Geophysics 61 (2007) 227-234 [Published Nov. 9, 2007], which paper is incorporated herein by reference in its entirety. Described in both provisionals and the earlier published article was an apparatus wherein a number of transmitting coils were orthogonally disposed one to the other as shown in FIG. 1 herein, the transmitting coils including paired horizontal coils 101a and 101b, and crossed vertical coils 102 and 103. The apparatus further included a plurality of circular receiving coils, these receiving coils 110 incorporated into the frame supporting the transmitting coils, and disposed in symmetric pairs as illustrated in FIG. 1A (for an exemplary single symmetric pair of receiving coils 1a and 1b), and FIG. 1C (for an exemplary array of eight symmetric pairs of receiving coils).
The described system was developed for determining the depth, size, and shape of buried metallic objects. The primary purpose of that system was to discriminate between intact, unexploded ordnance (UXO) and harmless metallic scrap. The system could also be used to locate buried metallic pipes or cables, as well as archeological objects or any metallic object of interest. The system of those prior provisionals afforded greatly enhanced detection capabilities compared to that obtainable by prior art methods through the novel configuration of the sensors of that system.
The operation of that system (and the one of this invention, as well) is based on the well known principles of electromagnetic (em) detection of metallic objects, usually referred to as metal locators or treasure finders. In all of these systems a source of changing magnetic field is provided. This is usually a loop of wire carrying a changing electric current. This source, usually called the transmitter, produces the changing magnetic field which pervades the volume of space near the transmitter. This is usually referred to as the primary field. The changing magnetic field produces an electromotive force, through Faraday's Law, in any nearby electrically conducting object. This in turn causes electric current to flow in the object. These induced currents in turn produce their own magnetic fields in their vicinity, called secondary fields. The detection of these secondary fields provides evidence of the presence of the metallic object.
The basic design challenge for these system is to provide a means to measure the secondary fields and relate them quantitatively to the location and parameters (size, shape and metal content) of the object (hereinafter called the target). The sensor used to measure the secondary field is usually called the receiver.
Almost all such systems use a single loop of wire for the transmitter and a second loop as the receiver. A magnetic field passing through a multi-turn coil of wires produces a voltage across the terminals of the coils which voltage is proportional to the time rate of change of the magnetic field. (This is the same voltage previously referred to as the electromotive force in Faraday's Law). Such a coil can be calibrated in a known field and this provides the required receiver for the secondary fields produced by the target.
A further theoretical and technical refinement is that the shape of the target may be deduced from knowledge of the currents induced in a target for different orientations of the transmitted field (also called the inducing field). To a high degree of approximation the induced currents in a metallic object can be represented by equivalent small loops of current. In general, any metallic object can be represented by three orthogonal current loops that are excited by incident fields aligned with the axes of these loops.
The equivalent strength of these principal current loops producing the secondary fields are called the principal moments of the target. These moments divided by the inducing field at the position of the induced current loop are called the polarizabilities of the target. It is important to note that these polarizabilities are a property of the target and thus serve to characterize it.
FIG. 4 illustrates how these polarizabilities characterize different shapes. In these examples, the principal polarizabilities are described by arrows (vectors) drawn perpendicular to the equivalent current loops in the body whose lengths are proportional to the strength of the induced moments. Smith and Morrison, Estimating Equivalent Dipole Polarizabilites for the Inductive Response of Isolated Conductive Bodies, (2004), IEEE Trans. Geosci. Remote Sens. 42 (6), 1208-1214, and Smith et al, Optimizing Receiver Configurations for Resolution of Equivalent Dipole Polarizabilities in Situ. (2005), IEEE Trans. Geosci. Remote Sens. 43 (7), 1490-1498 (both references incorporated herein by reference), have described the means of estimating the principal polarizabilities of a target using a system having three orthogonal transmitters and a multiplicity of receivers. In short, it was found that a configuration using three orthogonal transmitter loops and eight receiver pairs as shown in FIG. 1 to be sufficient to recover the position of the target and its three principal polarizabilities.
A design challenge for a practical system is to find a means to accurately measure the small secondary fields from the target in the presence of the much larger inducing field from the transmitters. This is called the dynamic range problem. One popular method of doing this is to run the transmitter in the so called transient or time domain. Briefly this means setting up the primary field with a steady current in the transmitter, and then abruptly shutting it off. The rapidly changing magnetic field produces the desired induced currents in the target and these induced currents slowly decay. The decaying secondary fields are thus measured by the receivers in the absence of the much larger primary fields.
Fundamentally any receiver is limited electronically in the range of voltage it can accommodate. During the shut-off of the transmitter, the induced voltage in the receiver coil may be several hundred volts while the induced voltage from the target secondary fields may be measured in fractions of a micro-volt. In a practical system the ratio of voltages measured over the complete range of the primary field would be an impossible 109 or higher. In principle, measuring in the off-time, when the high voltages associated with shut-off have dissipated, allows the full sensitivity and range of the receiver to be utilized to measure the desired secondary fields.
One suggested approach to get around this problem was to use receivers positioned in space a distance from a transmitter so as to not have any generated primary field from the transmitter pass through them. In other words, the receivers were placed such that there was no component of primary field from the transmitter along the axis of the receiver coil. This is referred to as a null-coupled configuration. This approach has also been used in some traditional metal locators which use a continuous sinusoidal current in the transmitter (so called frequency domain systems). The practical problem is that it is mechanically difficult to position the receiver in a true null position. The minutest deformation of the support structure by thermal expansion or vibration introduces large amounts of primary field and compromises the desired sensitivity of the receiver.
Unfortunately these transient systems are not completely immune from the dynamic range problem. While in principle the receiver should only see the secondary fields in the off-time of the transmitter, the fact is that the physical device comprising the receiver, the coil and its amplifiers are still subject to the primary transient and thus experience huge voltages just before they are expected to function at full null signal sensitivity.
In practice it was found (as disclosed in Provisional Application 60/975,911) that a combination of transient measurements employing null coupled receivers of a specialized arrangement can greatly overcome this problem. The system of that prior provisional had three orthogonal loop transmitters as shown in FIG. 1, and eight pairs of receivers (called the vertical moment transmitter) deployed in a symmetric pattern in the planes of the upper and lower horizontal transmitter loops. Any two receivers so located in these two planes whose centers lie on a diagonal passing through the geometric center of the volume defined by the two horizontal coils (101a and 101b) and the two vertical coils (102 and 103) see exactly the same primary field when any of the three transmitters are activated [Smith et al (2007), supra]. When these paired receivers are connected in series opposition to an amplifier, the amplifier sees no signal during the primary transient and has no problem with dynamic range in measuring secondary fields which are not symmetric with respect to the receiver pair—as for example the fields from the target. With this configuration, the location and orientation of the three principal polarizabilities of a target can be recovered from a single position of the transmitter-receiver system.
In one embodiment of that system, the transmitting and receiving coils were mounted to a rollable cart. The system itself included a current source, the transmitter array, a data acquisition module and a processing display module, all of which were mounted to the same rollable cart. In one embodiment the current source was derived from DC batteries carried on the cart. The transmitting coils 101a, b, 102 and 103 were carried by and contained within their respective non-conductive frame elements (see FIG. 1), the number of turns the same for each coil. In one embodiment approximately 30 turns of #10 AWG aluminum wire were use in forming the coils. In operation each coil was pulsed in sequence so that the transients from each transmitter were recorded independently.
As depicted in FIG. 1, the system array is provided in the form of a cube. The size of the cube is not critical, but it should not be so large as to cause loss of portability. In one embodiment, the length of each square side was conveniently selected at 1 meter. With the windings carried within their non-conductive frames, each transmitting coil is electrically isolated from the other transmitting coils. The receiver coils are likewise formed from multi-turns of wound wires which are electrically shielded and supported from above, in the case of horizontal coil 101a and from below in the case of coil 101b. In an embodiment, all of the coil components were electrostatically shielded one from the other, the frame of the system array formed from such non conductive materials as plastic, wood or aluminum.
The diagonally mirrored pairs of receiving coils of the earlier described system are wired together such that there their voltages sum to zero during the on-time phase for the transmitting coils. In the off-time, the secondary fields from the target are not the same at each of the receivers and the difference is directly proportional to the strength of the detected secondary field generated by a buried object, the summed output sent to the data acquisition and display components where the resulting voltage is amplified, processed by computer and the results displayed. In a typical interrogation, each transmitting coil can be sequentially cycled on, then off, with one set of readings recorded for each transmitter coil duty cycle, the two horizontal coils activated together. This produces 24 readings, representing the summed voltages associated with each of the 8 pairs of receivers, 8 readings for each of the three transmitter duty cycles.
Experience has shown that even this system suffers from the problem of inexact null coupling due to mechanical deformation of the structure, both in the manufacturing step, and in the deployment of the unit in the field, where the coil supporting structures themselves may deform as the unit is moved over the terrain to be interrogated. Further, the receiver coils are so close to the transmitter windings that significant leakage of the primary field onto the receiver circuit during the shut-off transient may occur and cause over range distortions of the recovered secondary transient wave form. Thus, there remains a need for an apparatus for reliably detecting buried metal objects including the location and shape of the objects, which method and apparatus does not suffer from the above drawbacks.